Anecdotal evidence over several years has been augmented recently by research evidence quantifying the relationship between strength of the hip girdle musculature and knee function.
An article published in the Journal of Orthopaedic & Sports Physical Therapy examined the relationship between hip strength and unilateral patellofemoral pain in female patients. This study specifically examined the presence or absence of hip girdle weakness in the involved (painful) lower extremity, compared with the uninvolved extremity.
It was normalized for body weight (so that force was expressed in a percentage of body weight rather than absolute force) and demonstrated that females presenting with unilateral patellofemoral pain also exhibited hip girdle weakness on the involved side. Specifically, the involved hip external rotators were 30% weaker, hip abductors 27% weaker, and hip extensors were 50% weaker. This study looked only at females, all of whom were relatively young, were experiencing moderate levels of disability, and had symptoms present for nearly 35 months on average. There were only 10 subjects in the symptomatic group, however, this study did compare the results to a control group of females that did not have patellofemoral pain. Despite these limitations, this study is generalizable in that the most frequently encountered patient with patellofemoral pain is a female in the age range of this study.
Why Is Hip Strength Important to Knee Function?
There is considerable debate about the cause and effect relationship, and even the causes of patellofemoral pain. In fact, there is considerable debate as to what it should be called. Recently, there has been a move among some to include all forms of anterior knee pain (pain along the front of the knee) into a diagnosis called “Anterior Knee Pain Syndrome.” A syndrome classification in this instance accounts for the several types of pain and clusters of signs and symptoms that can be discovered when examining a patient with pain along the front of the knee.
While specific causes differ, the relative risk factors and signs of anterior knee pain are widely known. People are at risk for patellofemoral pain generally when a combination of factors lead to “abnormal tracking” of the patella in the groove of the femur, or thigh bone. The knee cap rides along this groove (or in an esoteric and technical debate some argue that the groove technically rides along the knee cap) as the knee bends and straightens during normal activities. Those with anterior knee pain usually have some kind of aligment issue – flat feet, an excessive angle of pull of the thigh muscles that is though to pull the patella too far to the side, or alignment of the hip joint that also results in this abnormal motion at the knee cap. What can also lead to this motion, however, is weakness in the hip girdle muscles. These muscles are instrumental in guiding the lower extremity in shock absorption. As these muscles contract to absorb the shock of the foot hitting the ground, or to hold the pelvis level while a person is standing on one leg while walking (stance phase of gait) or when attempting to slowly lower the body down a step while standing on the involved lower extremity, weakness allows an abnormal amount of pronation to occur from the ankle upward, leading to knee pain.
While it has not been established which is the egg and which is the chicken, addressing hip girdle strength problems appears to be a keystone in the recovery of pain-free knee function. When strength and motion are normalized, pain is controlled and healing begins. Of course, that doesn’t mean that hip girdle strengthening is all that is involved in the rehabilitation of anterior knee pain – problems with range of motion, flexibility, inflammation, balance/agility, and other lower extremity strength deficits must be addressed to provide a thorough rehabilitative program.